<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .search {
            width: 170px;
            margin: 100px;
            position: relative;
        }
        
        .con {
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0, 0, 0, 0.2);
            box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
            display: none;
        }
        
        .con::before {
            content: '';
            width: 0px;
            height: 0px;
            position: absolute;
            top: 28px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }
    </style>
</head>

<body>
    <div class="search">
        <div class="con"> 123</div>
        <input type="text" placeholder="请输入快递单号" class="jd">
    </div>
    <script>
        //快递单号输入内容时,上面的大号字体盒子(con)显示(这里面的字号更大)
        //表单检测用户输入:给表单添加键盘事件
        //同时把快递单号里面的值(value)获取过来赋值给con盒子(innerText)作为内容
        //如果快递单号里面的内容为空,则隐藏大号字体盒子(con)盒子
        var con = document.querySelector('.con');
        var jd = document.querySelector('.jd');
        jd.addEventListener('keyup', function() {
                if (this.value == '') {
                    con.style.display = 'none'
                } else {
                    con.style.display = 'block'
                    con.innerText = this.value;
                }

            })
            //当我们失去焦点 就隐藏盒子
        jd.addEventListener('blur', function() {
                con.style.display = 'none'
            })
            //获得焦点 就显示盒子
        jd.addEventListener('focus', function() {
            if (this.value != '') {
                con.style.display = 'block'
            }

        })
    </script>
</body>


</html>